Vertical sizing or classifying of a stream of composite material is known. The prior art is typified by U.S. Pat. No. 2,853,191 wherein a stream of material is fed to a first inclined flat screen with all but the coarsest of the materials of the composite passing therethrough to a second screen therebelow. The process continues through a plurality of screens dependent on the number of groups into which the material is to be classified. In said patent all of the screens are of the same mesh but the effective openings of the screens is varied by varying the angle of the mount of each screen so that lower screens are progressively more vertically inclined thus reducing the effective size of the openings therein.
The basic advantage of the vertical sizing concept in general is that a high volume stream can be quickly divided. Most of the division is accomplished at the point the material is introduced to the screens with the remaining lower portions of the screen primarily acting as a conveying medium to transfer the material out of the machine. The problem encountered by the device of U.S. Pat. No. 2,853,191 is that because of the angular orientation of the screens there is little retention time of particles on the screen. If any high volume classification is attempted, some particles will never contact the screen surface but will rather remain in layers and drop quickly down the incline without ever being tested by the screen for size. Thus, the device of said patent does not accomplish the basic advantage of vertical sizing, i.e., efficient high volume separation.
Further problems have been encountered in these types of devices because of the manner in which the screens are tensioned. Usually a plurality of spring loaded draw bolt type tensioning devices are used for each screen. However, since the screens are under severe tension, breakage of a draw bolt is not uncommon. When such occurs, the recoil of the spring and flying broken parts could cause injury to a person in the vicinity. The prior art of which we are aware does not provide any type of convenient yet safe tensioning device.
Nor has the prior art solved the problem of adequate tensioning. Because the screens of the prior art are straight, there are limits to the amount of tension which can be placed thereon. A more highly tensioned screen will more efficiently separate the material thereon.